The present invention relates to an apparatus and method for forming molded parts from a polymeric material by injecting the material into a multiple cavity mold. More specifically, the present invention relates to an apparatus and method for promoting uniform flow of polymeric material and uniform filling of all mold cavities in a multiple cavity injection molding apparatus. Another significant aspect of the invention is that it eliminates a requirement of supplemental or external heating of runner systems used in conventional multiple cavity injection molding operations. Moreover, the invention eliminates conventional trial and error methods of adjusting flow parameters in order to achieve uniform filling of multiple molding cavities.
Multiple cavity injection molding offers several advantages over single mold cavity injection molding. A significant advantage is the increased number of molded articles produced for every injection molding cycle. Consequently, an increase in the number of parts produced has the potential to increase overall productivity and reduce production costs.
However, potential problems also arise with the use of multiple cavity injection molding. These problems stem from the flow of a polymeric material through the injection molding system. Ideally, the flow and state of the polymeric material is uniform as it travels from a feed source or through a runner system, to each respective mold cavity. However, flow paths or runner systems become increasingly complex as the number of mold cavities increases. This complexity is detrimental for the following reasons. Complex runner systems associated with multiple cavity injection molding may contribute to pressure changes which can alter the flow rate. Temperature changes or variances thereof along such relatively long runner systems may increase the viscosity leading to a reduced flow rate. Premature curing of thermoset polymeric material or solidification of thermoplastic polymeric material can also potentially reduce flow rates and cause eventual blockage of flow.
Such flow problems directly affect the production of molded parts. Cavities that are not sufficiently filled may produce unacceptable parts. As a result, productivity decreases and operating costs may increase in order to produce a desired number of acceptable products.
Nonuniform curing of the molded articles affects the physical characteristics of the articles. Different cure states of a polymer give rise to different physical properties. If the polymer is not in a uniform state as the mold cavities are filled or, if the polymer cures at different rates at various locations within the mold cavities or the runners, there exists a potential that the physical characteristics of the molded articles will vary from part to part. Some characteristics may be undesirable leading to defective molded parts which can not adequately function in a manner for which they were designed.
Conventional multiple cavity injection molding systems generally produce a great deal of scrap. Rejected parts, underfill material, overfill material, flash material and polymer material frozen in runners or gates are not commercializable and thus become scrap. A large amount of scrap typically increases production costs, as more material is required to produce the desired articles. Productivity may also decrease, as longer periods of time are needed to clean and prepare the system between injection cycles.
Various approaches have been employed in an attempt to avoid the problems associated with multiple cavity injection molding. A common approach to ensuring that the polymer is in a proper melt state is the use of thermally insulated or hot runner systems, along with trial and error adjustment of molding conditions such as temperature, pressure, and injection speed.
A means for controlling melt flow and reducing scrap is described in U.S. Pat. No. 5,945,139. An apparatus and method is disclosed in that patent for introducing an uncured thermoset material through a series of runners and into a plurality of cavities. Material in the cavities is allowed to cure and material remaining in the runners is kept live in order to be used in a subsequent injection cycle.
Techniques for ensuring sufficient filling of mold cavities have also been disclosed. These methods may involve devices, which monitor the amount of material supplied to a cavity, such as that described in U.S. Pat. No. 5,149,547.
U.S. Pat. No. 4,120,921 describes a method of sequentially filling mold cavities by supplying ultrasonic energy to a gating device. Material is injected into a cavity to which the ultrasonic energy has been supplied until the cavity is filled. After the material is frozen, ultrasonic energy is supplied sequentially to the gates of the remaining cavities. The material in each cavity is allowed to freeze before energy is supplied to the next gate.
Flow control is addressed in U.S. Pat. No. 5,849,236 which discloses an apparatus employing control members that may be inserted into a conduit to varying degrees to provide balanced flow of thermoplastic material into each of a plurality of cavities.
Although satisfactory in some respects, many of the prior art techniques and equipment used for multiple cavity injection molding fail in varying degrees, to adequately address the problems associated with multiple cavity injection molding. Also, prior art techniques cover thermoset polymeric materials, which employed compression and/or transfer molding methods. In transfer and compression molding procedures, longer periods of time, eg. up to 50 minutes, are afforded for the melt flow to travel to the cavity and for the curing time inside the cavity, whereas thermoplastic polymeric materials can only be afforded relatively short travel and fill times, eg. 5 to 30 seconds. Therefore, external heating devices and ultransonic methods were found to be costly and ineffective. For instance, external heating of runners or control devices, as discussed in the prior art, involve additional costs in both the design and operation of the molding apparatus.
Accordingly, there still exists a need for an improved approach suitable for remedying at least some of the difficulties associated with multiple cavity injection.
In one aspect, the present invention provides a multiple cavity injection molding system that remarkably and consistently produces molded parts from injecting molten polymeric material simultaneously into a collection of mold cavities. The system comprises a feed source of molten polymeric material, a collection of mold cavities, a primary sprue for transferring polymeric material from the feed source, and a runner system for distributing the polymeric material from the primary sprue to the collection of mold cavities. The runner system utilizes a unique configuration and at least two turbulence inducing components that maintain a particular shear rate within the polymer as it exits the runner system and enters the mold cavities.
In another aspect, the present invention provides a runner system adapted for multiple cavity injection molding in which the runner system comprises a primary runner, first and second secondary runners, and a plurality of tertiary turbulence inducing runners, arranged in a particular configuration. Upon introducing a flowable material into a feed opening of the primary runner at a pressure sufficient to cause the material to flow through the runner system and exit through exit ports defined in the tertiary turbulence inducing runners, the material while in the tertiary turbulence inducing runners, is in a pseudo-turbulent state.
In a further aspect, the present invention provides a turbulence inducing runner adapted for incorporation in a runner system for a multiple cavity injection molding system. The turbulence inducing runner comprises a primary runner, first and second secondary runners in communication with the primary runner, and a plurality of turbulence inducing steps in communication with the first and second secondary runners.
In yet another aspect, the present invention provides a method for promoting uniform filling of multiple mold cavities. The method comprises a step of providing a collection of runner components, and also selecting at least two turbulence inducing components that are combined with the runners. The method further comprises the step of configuring together the runners and turbulence inducing components in such a manner as to form a runner system having a particular geometry that has been found to maintain a particular shear rate within the polymeric material when flowing through the runner system.